Granholm was rumored to be interested in the seat, and would have entered the race with strong name recognition in the state. Her decision not to run will intensify the focus on the Democratic side to Reps. Gary Peters and Dan Kildee. There's also speculation Democratic power broker Debbie Dingell, who is married to Rep. John Dingell (D-Mich.) is interested.
Levin announced earlier this month that he'll retire at the end of his sixth term, opening up a seat that Republicans believe they can put in play.
Multiple Republican members of the Michigan delegation could run, including Reps. Mike Rogers, Justin Amash and Dave Camp. None have yet made a bid official, although Camp is said to be interested.
Mitt Romney's niece, Ronna Romney McDaniel, has also expressed interest in the seat. She is the granddaughter of former Michigan Gov. George Romney.